The present invention relates generally to ceramic matrix composites, and more particularly to ceramic matrix composites reinforced with silicon carbide fibers and comprising additional glass and inorganic whisker phases Composites of this type are disclosed which exhibit greatly enhanced resistance to oxidation embrittlement.
The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics, and ceramics is now well known. The mechanism of strengthening of glass or ceramic bodies by fibers is considered to be that of load transfer by the matrix to the fibers through shear. This load transfer shifts stress from the glass or ceramic matrix to the relatively long, high modulus fibers, while the fibers at the same time may act to impede crack propagation in the matrix material.
Whiskers are thought to impart strengthening by a similar mechanism, but load transfer to whiskers by the matrix is more limited due to the limited length and aspect ratio of the whiskers Theoretically, a whisker which is sufficiently short will not be loaded to the breaking point by the matrix under stress, and therefore full advantage cannot be taken of the high strength of the whiskers.
Among the fibers and whiskers which have been suggested for use as reinforcement for non-metal matrix materials are silicon carbide, silicon nitride, alumina and carbon fibers or whiskers. For example, U.S. Pat. No. 4,324,843 describes SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is of alumino-silicate composition. U.S. Pat. No. 4,464,475 describes similarly reinforced glass-ceramics comprising barium osumilite as the predominant crystal phase, while U.S. Pat. No. 4,464,192 describes whisker-reinforced glass-ceramic composites of aluminosilicate composition.
A principal purpose for incorporating whisker reinforcement in ceramic matrix materials for high temperature applications is that of increasing the toughness of the material. A toughened ceramic material exhibits improved resistance to cracking failure from flaws sustained in use, offering the possibility of increased fatigue lifetime. As noted in U.S. Pat. No. 4,626,515, the addition of fiber reinforcement to glasses such as alkali-free alkaline earth aluminosilicate glasses can result in substantial strengthening, while whisker additions to those glasses were found effective to enhance the toughness of the glass.
A second reason for incorporating whiskers into such materials is to enhance the transverse or so-called "off-axis" properties of the composite. Many structural fiber-reinforced composites are of laminar type, i.e., the fiber reinforcement is preferentially disposed in layers within the material. The layers consist of fiber groups or arrays wherein the fibers are principally disposed in substantially parallel alignment in a single direction, giving rise to a "strong" axis in the material. Ceramic composites to be utilized in high-stress environments should exhibit strength properties which are more isotropic, i.e., not confined to the fiber axis, and it has been reported, for example in U.S. Pat. No. 4,615,987, that whisker additions can provide improved transverse strength properties.
Prospective uses for fiber-reinforced ceramic matrix composites such as described in these and other prior publications include structural applications in high temperature heat engines For these applications the materials must exhibit good strength and toughness at elevated as well as ambient temperatures. Temperatures in the range of 700.degree.-1000.degree. C. and highly oxidizing conditions (due to the high-temperature activity of oxygen) will probably be encountered.
An important problem which has been identified in silicon carbide reinforced ceramic matrix composites in this temperature regime is that of high temperature embrittlement. Hence, even though these composites are strong and tough at room temperatures, at temperatures of interest, i.e., 900.degree. C. or 1000.degree. C., the composites fail in a brittle, catastrophic manner, and often at stresses which are only 30 to 40% of the ultimate strength obtained at room temperature. While the exact mechanism of embrittlement has not been fully explained, oxidative deterioration of the fiber-matrix interface is the probable cause. See, for example, R. L. Stewart et al., "Fracture of SiC Fiber/Glass-Ceramic Composites as a Function of Temperature," in Fracture Mechanics of Ceramics, R. C. Bradt et al. Ed., Volume 7, pages 33-51, Plenum (New York) 1986.
It is known that, in ceramic composites, the matrix has a significantly lower strain-to-failure than the silicon carbide fiber reinforcement phase. Therefore, under strain, microcracks develop in the composite matrix long before the ultimate fiber-enhanced failure strain of the composite is reached. Typically, the matrix microcrack stress is only about 30 to 50% of the ultimate strength of the composite.
Composites comprising silicon carbide fibers such as the frequently used Nicalon.RTM. silicon oxycarbide fibers derive their toughness from a graphitic layer which develops in situ adjacent the fiber surface during composite manufacture, and which controls fiber-matrix bonding in the composite. Below the microcrack stress point this layer is protected by the matrix from the high-temperature oxidizing environment, but above that point the environment has access to the fiber-matrix interface and, at temperatures of 900.degree.-1000.degree. C., oxidation of this layer occurs very quickly. After such exposure, the composites invariably fail in a brittle rather than in a tough or "graceful" manner.
Since the avoidance of brittle failure is critical to the successful high-temperature application of these materials, a number of solutions to the oxidation embrittlement problem have been proposed One such solution is to simply increase the microcrack stress point of the matrix, for example by the introduction of a strengthening whisker phase. This can be achieved, but brittle failure at high temperatures is still observed in the resulting materials.
Another approach is to employ additional fiber coatings to protect or replace the graphitic interface that imparts desirable fracture behavior. For example, U.S. Pat. No. 4,642,271 suggests that the high-temperature strength and toughness of a composite comprising an SiO matrix and SiC reinforcing fibers may be significantly improved by coating the fibers with boron nitride. However, these and other coatings add cost to the composite system and have not yet been shown to be fully effective in preserving composite performance at very high temperatures.
It is also known to add B.sub.2 O.sub.3, alone or as a mixed glass, to silicon-carbide-fiber-reinforced lithium alumino-silicate glass-ceramic composites to reduce so-called "pipeline" oxidation of the graphitic fiber/matrix interface. While some improvements in oxidation protection have been achieved in this fashion, the large B.sub.2 O.sub.3 additions required result in matrix blistering at high temperatures, and also lower the practical use temperatures of the composites to about 800.degree. C.
Therefore there remains a need to further improve the performance characteristics of fiber-reinforced ceramic matrix composites, particularly at high temperatures and under oxidizing conditions.
It is a principal object of the present invention to provide novel fiber-reinforced glass-ceramic matrix composites which exhibit improved resistance to embrittlement under adverse high temperature conditions.
It is a further object of the invention to provide novel fiber-reinforced ceramic matrix composites which exhibit improved resistance to stress failure in a hot oxidizing environment.
It is a further object of the invention to provide a method for making silicon carbide-reinforced glass-ceramic matrix composites which provides products of improved strength and/or toughness at high temperatures.
Other objects and advantages of the invention will become apparent from the following description thereof.